In xsk_socket__create(), the libbpf_flags field was not checked for
setting currently unused/unknown flags. This patch fixes that by
returning -EINVAL if the user has set any flag that is not in use at
this point in time.

Fixes: 1cad0788 ("libbpf: add support for using AF_XDP sockets")
Signed-off-by: NMagnus Karlsson <magnus.karlsson@intel.com>
Reviewed-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>